The invention relates to a system for automatically controlling the operation of a pulp bleachiing operation by means of one or more chromatic sensors employing one or more wavelengths of light for monitoring the reflectances of the pulp slurry during the bleaching process.
It is an established practice in the pulp processing industry to optically monitor samples of pulp slurry in order to control either manually or automatically certain factors in the treatment of the pulp at different stages of the processing operation. Many of the optical measuring devices in these prior systems utilize the light reflectance characteristics of a sample of pulp slurry in one form or another or at one stage or another. Some devices simply measure the brightness of the pulp slurry at one waveband. In other devices the reflectance is measured through polarizing systems which determine gloss. In still other devices the luminosity of the pulp slurry is optically measured.
In general such systems are of two types: the optical information obtained is used to control changes upstream from the measurement, which is called a "feedback" system, or the information is used to control treatment downstream from the measurement, which is called a "feed forward" system. In the present system feedback and/or feed forward control may be employed.
Control of pulp solution viscosity, as measured by TAPPI Standard T230 or some other method, is desirable since this property is related to the strength potential of the pulp and to its beating characteristics. Beating is a mechanical treatment used to enhance fiber bonding and paper strength. A very high viscosity pulp is strong but requires high inputs of energy to beat the pulp to develop its potential physical strength. On the other hand, very low viscosity pulp has low strength potential and requires less beating energy input. The most desirable pulp characteristics for papermaking fall somewhere between these extremes. Pulp with very high viscosity can be bleached with hypochlorite to reduce its viscosity and provided this is not overdone there is only a slight loss in strength potential but a relatively large decrease in the power required to beat the pulp. Therefore, viscosity control allows the papermaker to obtain the desired balance between strength and power consumption in the beating operation.
Although optical control systems have been available to control pulp brightness in the past, so far as we are aware, optical control systems have never been used during bleaching procedures to control both the solution viscosity and brightness of the pulp simultaneously. Viscosity control is an important consideration since unbleached pulp which is to undergo a bleaching operation can vary over a wide range of viscosities, depending on the nature of the preceding cooking treatment. Not only is it desirable to control the bleached brightness, but to prevent undue reduction of the viscosity of the pulp as a result of the bleaching treatment. Pulps which have too low a viscosity may have to be culled. It is also undesirable that the pulp have too high a viscosity since this requires extra beating of the pulp in order to develop the desired strength properties in the pulp. It is present day commercial practice to control bleached pulp brightness by making adjustments to chemical additions based on feedback information from manually prepared and measured brightness sheets. A degree of viscosity control, almost solely to avoid low viscosity levels, is practiced, but based on feedback information from manually tested samples.
Bleaching of wood pulp customarily involves a sequence of steps designed to increase the brightness of the pulp, while minimizing degradation of the pulp by the chemical bleaching agents employed. Most bleaching processes employ an initial chlorination stage. The use of hypochlorites in a succeeding stage is often considered judicious since it increases the brightness of the pulp economically but must be carefully controlled to avoid undue degradation of the strength potential of the pulp. The use of chlorine dioxide in a further successive or final stage of the bleaching process is now widely used since it also has advantageous properties in increasing brightness, without undue loss of viscosity.